This invention relates to metal alloys and more particularly to titanium-nickel based alloys having shape change memories.
Certain of the Nitinol (nickel-titanium based) alloys are noted for their shape memory recovery when they are heated through their critical recover-temperature range or transition temperature range (TTR) following plastic straining. The reproducibility of the shape change memory property is most severely tested in multiple cycling, where a given titanium-nickel base alloy specimen is strained plastically within the limits for "memory" recovery range (about 8% axial or outer fiber strain) followed by inducing shape recovery by heating the specimen through the transition temperature range (TTR).
Prior efforts to attain shape memory perfection have been attempted by others. For example, note U.S. Pat. No. 3,652,969, entitled "Method and Apparatus for Stabilizing and Employing Temperature Sensitive Materials Exhibiting Martensitic Transitions," which issued to J. Willson and D. Carey on Mar. 28, 1972. That patent only addresses micro-twinning and more specifically micro-twin orientation. While the maximization, refinement, and orientation of micro-twins is very important, these factor are not sufficient to provide the maximum magnitude and reproducibility of shape change.